The
Millen frequency standard probably didn't see much use in the
ham shack as it did it the laboratory environment. It has an adjustable
RF output in intervals of 10, 25, 100 and 1000 Kcs with useful
signal strength to 50 Mcs. It also incorporates an internal 800
cycle audio generator/modulator and a detector with phone jack.
The unit runs from a very stable helium filled 1000 Kcs vacuum
quartz oscillator and uses synchronized multivibrator oscillators
to create the 10, 25 and 100 Kcs markers.

I've noticed that there appeared to be different designs produced
which changed the RF output and modulator input connector types.
The variation model 90505 appeared in the 1952 ARRL Handbook used
single ceramic base quick release binding posts. As you can see
this unit has both black bakelite banana jack and standard SO-239
connectors. At first I thought that the SO-239's were add-ons
by a previous owner, but after disassembling the unit the workmanship
appears to be very professional and I would surmise it was an
original Millen variation.